No, when it says "weapon attack," that means you can do it either as a melee or a ranged attack. That's how it's always worked with most Battle Master maneuvers, for example. It is clearly not meant to exclude anything that already worked with "melee weapon attack."
My point is that "weapon attack" excludes Unarmed Strike, because they are not weapons. But they can be used anywhere it says "melee weapon attack" because that is what the explicit exception is for them. They are not weapons, so can't be used for "weapon attack" triggers. They are "melee weapon attacks" so can be used for those triggers.
I agree that any weapon can be used for "weapon attack" triggers. Key word weapon, ranged or not, but they are two different things.
It’s unclear. The rules don’t explicitly define “weapon attack.” It seems to be a use of natural language, which could either be a shorthand for “melee weapon attack or ranged weapon attack,” or could be literally referring to an attack made with a weapon. Seems worth tweeting Crawford about if one cares what the RAI is on this matter.
I think they were very clear and explicit in their use of language for these things. I don't personally require any further clarification on the matter because I think it is clear.
When it says "weapon attack" to trigger something, your attack has to use a weapon, ranged or melee. Unarmed strikes aren't weapons, so don't trigger those conditions.
When it says "melee weapon attack" then it can be triggered by anything that counts under that category, which includes melee weapons and unarmed strikes per the specific exception in that entry.
It's just like squares and rectangles. All Unarmed Strikes are included under the Melee Weapon Attack bucket (because what else could they be?), but not all "weapon attacks" allow Unarmed Strikes (because they're not weapons).
I dont' think it's really that confusing in the language they use. The only confusion is that the original printings have Unarmed Strikes on the weapons table.